The present invention relates to a propylene random copolymer composition. Particularly, the present invention relates to a propylene polymer composition suitable for forming a heat sealing film usable as a film for packaging cigarettes, caramels, etc., the composition being obtained by blending at a suitable ratio a propylene polymer and a propylene random copolymer that have specific properties.
As is well known, a propylene homopolymer or copolymer (these will hereinafter be generically referred to as "propylene polymers" unless otherwise specified) has excellent properties, which allow its use to be developed in a wide range of fields.
For instance, a propylene polymer is widely used to form a packaging or wrapping film. Since polypropylene has a relatively high melting point, when a polypropylene is to be applied to such use, in order to improve heat sealing ability at low temperatures, a propylene random copolymer is generally used, in which ethylene or an .alpha.-olefin having a carbon number of 4 to 20 is copolymerized with propylene.
Although a packaging or wrapping film comprising a conventional propylene random copolymer has superior transparency and scratch resistance to a film comprising a low-density polyethylene, its heat sealing ability at low temperatures is still insufficient. In order to further improve low-temperature heat sealing ability a copolymer is known in which an .alpha.-olefin having a carbon number of 4 to 20 is copolymerized in an increased amounts. Although such an increase enables the resultant film to have an improved low-temperature heat sealing ability, its blocking resistance deteriorates and its rigidity is lowered.
Thus, in order to improve low-temperature heat sealing ability and blocking resistance, the advent of a propylene random copolymer having a low melting point and containing only a small mount of component that causes blocking, is desired.
Hitherto, a propylene has been produced by employing, in general, an olefin polymerization catalyst comprising a titanium compound and an organic aluminum compound. In recent years, however, proposals such as the following have been made: proposals concerning a novel Ziegler-type olefin catalyst comprising a metallocene compound and alumoxane (Japanese Patent Laid-Open Nos. 58-19309, 60-35007, 61-130314, 63-295607, 1-275609, 1-301704, and 2-41303); proposals concerning a propylene random copolymer and a method of producing the same using such an olefin polymerization catalyst (Japanese Patent Laid-Open Nos. 62-119215, 1-266116, 2-173014, 2-173015, and 2-247207); and proposals concerning use of such a copolymer as a heat sealing agent (Japanese Patent Laid-Open No. 2-173016).
It is possible with these proposals to obtain a propylene random copolymer having a low melting point and containing only a small amount of component that causes blocking. However, the proposals have some problems. For example, the molecular weight distribution is such that it is too narrow to form a film in practice, or causes surface roughness which renders the film unusable.